Background/Rationale: Current guidelines recommend screening for drug use disorders, and referral of patients with drug disorders to substance use disorder (SUD) specialty treatment (SAMHSA, 2005; Sullivan & Fleming, 1997; VA/DoD Evidence-based Clinical Practice Guideline Working Group, 2001; Veterans Health Administration, 2005). However, current guidelines do not recommend a specific instrument because no instrument is appropriate for this purpose (Berman et al., 2005; McPherson & Hersch, 2000; Tiet et al., 2008). There is no instrument that is appropriate for drug-screening purposes that is suitable for VA primary care patients that is brief enough and has been validated (McPherson & Hersch, 2000; Tiet et al., 2008). Objectives: There are three primary aims for this study: Aim 1 is to revise, shorten, and examine the sensitivity and specificity of six existing drug screening instruments for patients in the primary care (PC) settings and to provide six revised, shorter, and more practical drug screening instruments to be used in PC settings. .The revised instruments will be examined against how well they detect (a) drug-related disorders, and (b) drug- related consequences and problems in patients who may not meet diagnostic criteria for a drug use disorder. Aim 2 is to develop a new instrument from combining items across the existing instruments, and the best instrument with the optimal cut-point (considering the relative number of false positives and false negatives) will be identified from comparing the six revised, shortened, existing instruments and the newly constructed instrument. Aim 3 is to examine how well the six revised and one newly developed drug screening instruments perform in detecting drug use disorders and drug-related problems among PC patients with and without Post- Traumatic Stress Disorder (PTSD). Methods: Patients will be recruited from PC clinics at the VA Palo Alto HCS over a 24-month period. Six existing screeners will be revised, shortened, and examined for their psychometric properties, and a new instrument will be developed from choosing and comparing items across existing instruments. The best instrument will be identified from comparing psychometric properties of the six revised, shortened, existing instruments and the newly developed instrument. The criterion measure (gold standard) for a drug use disorder will be considered present when a drug use diagnosis is detected by a structured diagnostic interview (CIDI, WHO, 1990) conducted by the study team. The Inventory of Drug Use Consequences (InDUC; Tonigan & Miller, 2002) will be used as the second criterion (gold standard) for measuring drug-related problems for PC patients who may not meet diagnostic criteria for a drug related diagnosis. Tree based signal detection using the receiver operating characteristic (ROC), multivariate logistic regression analyses, and area under the ROC curve (AUC) analytical techniques will be conducted to identify the best screening instrument. Impact: A brief and practical drug screening instrument will have wide-ranging benefits to PC patients with or without PTSD. Early detection and intervention are associated with better PTSD and drug-related outcomes, and reduction of health care cost for the VHA. A validated screener will be the foundation for future studies to identify patients who need differential intensities of interventions to inform clinical practice. Long-term potential impact of the proposed study is information that can lead to the development of VA practice guidelines for screening and managing patients with drug-related disorders or problems in the PC system, and to make detection, assessment, treatment and management or referral for treatment of PTSD patients with drug-related problems an integral part of the treatment system in the VA.